The connection between an oscilloscope probe and a device under test (DUT) is often unreliable due to motion of the probe, e.g., if held by hand, motion of the DUT, e.g., vibration or thermal expansion, or both. The presence of a faulty connection for the “active” lead, e.g., signal lead of a probe to the DUT, is usually easy to determine if the registered signal is dramatically different from the expected signal.
However, the presence of a faulty connection for the “cold” lead, e.g., ground lead of a probe or negative lead of a differential probe connected to the DUT ground, is much more difficult to determine. This is because there is usually a low-frequency ground connection between the oscilloscope and the DUT through grounding power cords, other probe channel grounds, etc., and sometimes a high-frequency connection through capacitive coupling across the faulty connection. Thus, only a band of frequencies is typically misrepresented in the acquired oscilloscope record, which often causes subtle errors that are easy to miss by a user.
Oscilloscope users have traditionally dealt with this unreliability by either wiggling the probe to see if anything changes in the signal or temporarily disconnecting the probe from the oscilloscope and measuring the resistance from the probe ground to the DUT ground with an ohmmeter.
While the “probe wiggling” approach is generally quick and intuitive, this approach is rather heuristic in nature and often causes a good connection to become bad. While the “temporary disconnecting” approach indeed checks the probe ground to DUT ground connection, this approach requires manual intervention and also adds wear to the probe connector.